A team led by professor Yadong Yin at the University of California, Riverside formulated a light-sensitive coating material for rewritable light-printing. The material is made from doped titanium dioxide (TiO₂) nanocrystals that are synthesized in a single-step process.

When the nanocrystals are applied to a glass or paper substrate, they form a uniform coating that can be written on with UV light. The writing can be erased by exposing it to oxygen.

The method used to produce the light-writable coating relies on standard, nontoxic starting materials that are compatible with a range of other materials. The light-sensitive material supports high-resolution printing and writing, delivers stable color at ambient temperature, and demonstrates excellent writing and erasing reversibility. The nanocrystals could provide a low-cost, reusable writing and printing option for applications such as optical data recording, smart windows, and oxygen sensors, according the researchers.

Nanocrystalline TiO₂, a semiconductor, changes from white to black when it is irradiated with UV light, due to charge separation and the reduction of titanium atoms by photoexcited electrons. However, the color change requires substantial light energy, and the coloration vanishes instantaneously upon exposure to air. Oxygen in the air re-oxidizes the titanium and causes it to become transparent once again.

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A graphical abstract rendering of the rewritable, UV-sensitive surfaces made from doped (TiO₂) nanocrystals. University of California, Riverside researchers used the TiO₂ nanocrystals to develop a coating material that can be written on using UV light and erased again using oxygen. The development supports color-switching applications, as well as optical data recording, smart windows, and oxygen sensors. Courtesy of Angewandte Chemie International Edition (2022).

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To make the color change last longer, the researchers doped the TiO₂ with nitrogen obtained from urea and decorated the nanocrystals with diethylene glycol (DEG). When DEG was added as a solvent, it scavenged the excess electron holes, delaying reoxidation and the material’s return to a transparent state.

The researchers explored two light-writing methods using the coating. They produced patterns and printed text by illuminating a coated paper or glass substrate through a photomask, and they produced freehand writing on the substrates using a laser pen. Both methods resulted in a high-contrast pattern that remained stable for many hours. The patterns and text can be erased by heating, or allowed to fade slowly via oxidation. The researchers said the light-printing can be made to last longer by covering the film surface with a protective layer of nontoxic polymer to reduce the material’s exposure to oxygen.

A strong light source is not necessary to use the reversible light-writing system. Just 30 sec of illumination with a light source at less than 400 nm is all that is needed to produce writing on the UV-sensitive surfaces coated with the doped TiO₂ nanocrystals. Lamps in the power range of LEDs are sufficient to produce a high level of contrast in the material, according to the researchers.

They demonstrated that up to 50 write-erase cycles could be completed using the system, without any notable loss of contrast.

The highly reusable system could be applied in fields where reusable/rewritable surfaces are required. It could be used to print daily transport tickets and information boards, store data, or print signals from optical sensors.

In response to UV irradiation and oxygen exposure, the rewritable paper/glass substrate fabricated using N-doped TiO₂ nanocrystals exhibited strong writing and erasing reversibility. These capabilities, along with the material’s color stability, additionally make a potential light-writing method based on the solution a useful option for color-switching applications.

The research was published in Angewandte Chemie (www.doi.org/10.1002/anie.202203700).

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